Ted Hughes expected the reader to find his or her own interpretation of his poetry and as such, his subjects are sometimes difficult to follow. In Relic, he uses the sea, a long time fascination of his, to make his point about the continuing cycle of life as "Time in the sea eats its tail."
The sea is significant in relation to the repeating pattern as the depths of it hold many unsolved mysteries as mankind has yet to discover all that it holds where "The deeps are cold." The sea is also an unfriendly place where survival is key and "In that darkness camaraderie does not hold" as creatures prey on each other to survive.
The speaker is contemplating a bone that he has found and is led to wonder about any sea creature and how it will "flap for half an hour and turn to a crust" as it dies and makes its contribution to "Continue the beginning." In other words, life recycles and the sea can be proud of its " achievement" as life goes on despite the destructive nature of the sea and the creatures within it as" Nothing touches but, clutching, devours. "
The tone reveals that the sea is not a hospitable place and even the creatures that prey on other creatures also become nothing more than "Jaws / Eat and are finished and the jawbone comes to the beach" as they also become part of the recycling process.
The bones become representative of the sea itself as they serve as "a cenotaph" - a reminder, a monument; just as a trophy reminds us of a noble animal which has served its purpose. It's as if the sea is itself the predator as it " thrives," and makes use even of those seemingly useless creatures "Indigestibles" and goes on, absorbing those "That failed far from the surface."